Mosaic Control

Losses to tobacco mosaic virus (TMV) hit an all time record in 2000 with an estimated loss of $1,527,000! The early development of the disease suggests that initial infection of the seedlings occurred in the greenhouse. It is unclear how and why mosaic was observed in so many greenhouses in 2000. TMV to date has not been shown to be seed borne. The change to greenhouse production of seedlings many have magnified a minor problem into a major one. Just one plant within a large greenhouse that is TMV positive can have a devastating effect on the quality of seedlings grown within that greenhouse. Although losses were low in 2008, careful sanitation is needed in all years to prevent TMV.

Growers should not reuse trays from any greenhouse that had TMV the previous year. Plant roots grow through the tray and it would be impossible to remove all root fragments and sterilize the trays to assure they were TMV free. I would expect that the transmission of TMV to new seedlings would be very low, however you only need one infected plant per greenhouse to spread the virus during mowing. If trays are to be reused consider the use of TMV resistant cultivars such as CC27, CC 37, NC 102, NC 297, NC 471, RGH4 or SPH20 (see variety table).

Remember, tobacco mosaic is caused by a virus that is very easily spread by hand or machinery. If workers do not wash their hands with abrasive soap or dip them in milk every 30 minutes while handling transplants, the virus can be introduced into the field and very efficiently spread within the field. Mosaic can be spread at any time in the growing season; it is commonly spread by hand topping. Mosaic does not kill the plant but produces symptoms which range from a mild mottling on the leaves to distortion and "mosaic burn" on the leaves. Mosaic infection early in the season results in stunted, low yielding plants. If mosaic burn (dead areas in the leaves) occurs, both yield and quality are reduced. Even without severe symptoms, losses to mosaic are expensive, thus making tobacco mosaic one of the most important diseases.

The following points should be considered to help control mosaic:

1. Rotate tobacco fields.
2. Do NOT use tobacco products when working in the plant beds or in greenhouses, during transplanting, or during topping.
3. Do NOT cover or carry tobacco transplants on old or possibly contaminated tobacco sheets.
4. When clipping transplants in beds or greenhouses, disinfect the underside of the mower with chlorine bleach mixed 1:1 with water immediately after each clipping.
5. Wash hands with abrasive hand soap (such as "Lava") or dip them in milk before handling plants. Repeat every 30 minutes.
6. Before first cultivation, remove plants showing mosaic symptoms.
7. Avoid unnecessary cultivations.
8. Complete layby cultivation before plants are tall enough to touch equipment under tractor.
9. Follow root and stalk destruction recommendations as soon after harvest as possible.
10. Most mosaic infections begin in fields from previous crop residues!
11. 10. Use resistant varieties, such as CC27, CC 37, NC 102, NC 297, NC 471, RGH4 or SPH20 where mosaic is severe or rotation is not practiced.